At all times, PostgreSQL
maintains a write ahead log (WAL) in
the pg_wal/ subdirectory of the
cluster's data directory. The log records every change made to
the database's data files. This log exists primarily for
crash-safety purposes: if the system crashes, the database can be
restored to consistency by “replaying” the log entries made since the
last checkpoint. However, the existence of the log makes it
possible to use a third strategy for backing up databases: we can
combine a file-system-level backup with backup of the WAL files.
If recovery is needed, we restore the file system backup and then
replay from the backed-up WAL files to bring the system to a
current state. This approach is more complex to administer than
either of the previous approaches, but it has some significant
benefits:

We do not need a perfectly consistent file system backup
as the starting point. Any internal inconsistency in the
backup will be corrected by log replay (this is not
significantly different from what happens during crash
recovery). So we do not need a file system snapshot
capability, just tar or a
similar archiving tool.

Since we can combine an indefinitely long sequence of
WAL files for replay, continuous backup can be achieved
simply by continuing to archive the WAL files. This is
particularly valuable for large databases, where it might
not be convenient to take a full backup frequently.

It is not necessary to replay the WAL entries all the
way to the end. We could stop the replay at any point and
have a consistent snapshot of the database as it was at
that time. Thus, this technique supports point-in-time recovery: it is possible to
restore the database to its state at any time since your
base backup was taken.

If we continuously feed the series of WAL files to
another machine that has been loaded with the same base
backup file, we have a warm
standby system: at any point we can bring up the
second machine and it will have a nearly-current copy of
the database.

Note

pg_dump and pg_dumpall do not produce
file-system-level backups and cannot be used as part of a
continuous-archiving solution. Such dumps are logical and do not contain enough
information to be used by WAL replay.

As with the plain file-system-backup technique, this method
can only support restoration of an entire database cluster, not a
subset. Also, it requires a lot of archival storage: the base
backup might be bulky, and a busy system will generate many
megabytes of WAL traffic that have to be archived. Still, it is
the preferred backup technique in many situations where high
reliability is needed.

To recover successfully using continuous archiving (also
called “online
backup” by many database vendors), you need a
continuous sequence of archived WAL files that extends back at
least as far as the start time of your backup. So to get started,
you should set up and test your procedure for archiving WAL files
before you take your first
base backup. Accordingly, we first discuss the mechanics of
archiving WAL files.

25.3.1. Setting Up WAL
Archiving

In an abstract sense, a running PostgreSQL system produces an indefinitely
long sequence of WAL records. The system physically divides
this sequence into WAL segment
files, which are normally 16MB apiece (although the
segment size can be altered during initdb). The segment files are given
numeric names that reflect their position in the abstract WAL
sequence. When not using WAL archiving, the system normally
creates just a few segment files and then “recycles” them by
renaming no-longer-needed segment files to higher segment
numbers. It's assumed that segment files whose contents precede
the last checkpoint are no longer of interest and can be
recycled.

When archiving WAL data, we need to capture the contents of
each segment file once it is filled, and save that data
somewhere before the segment file is recycled for reuse.
Depending on the application and the available hardware, there
could be many different ways of “saving the data
somewhere”: we could copy the segment files to an
NFS-mounted directory on another machine, write them onto a
tape drive (ensuring that you have a way of identifying the
original name of each file), or batch them together and burn
them onto CDs, or something else entirely. To provide the
database administrator with flexibility, PostgreSQL tries not to make any
assumptions about how the archiving will be done. Instead,
PostgreSQL lets the
administrator specify a shell command to be executed to copy a
completed segment file to wherever it needs to go. The command
could be as simple as a cp, or it
could invoke a complex shell script — it's all up to you.

To enable WAL archiving, set the wal_level
configuration parameter to replica
or higher, archive_mode to
on, and specify the shell command
to use in the archive_command
configuration parameter. In practice these settings will always
be placed in the postgresql.conf
file. In archive_command,
%p is replaced by the path name of
the file to archive, while %f is
replaced by only the file name. (The path name is relative to
the current working directory, i.e., the cluster's data
directory.) Use %% if you need to
embed an actual % character in the
command. The simplest useful command is something like:

which will copy archivable WAL segments to the directory
/mnt/server/archivedir. (This is
an example, not a recommendation, and might not work on all
platforms.) After the %p and
%f parameters have been replaced,
the actual command executed might look like this:

The archive command will be executed under the ownership of
the same user that the PostgreSQL server is running as. Since the
series of WAL files being archived contains effectively
everything in your database, you will want to be sure that the
archived data is protected from prying eyes; for example,
archive into a directory that does not have group or world read
access.

It is important that the archive command return zero exit
status if and only if it succeeds. Upon getting a zero result,
PostgreSQL will assume that
the file has been successfully archived, and will remove or
recycle it. However, a nonzero status tells PostgreSQL that the file was not archived;
it will try again periodically until it succeeds.

The archive command should generally be designed to refuse
to overwrite any pre-existing archive file. This is an
important safety feature to preserve the integrity of your
archive in case of administrator error (such as sending the
output of two different servers to the same archive
directory).

It is advisable to test your proposed archive command to
ensure that it indeed does not overwrite an existing file,
and that it returns nonzero status
in this case. The example command above for Unix
ensures this by including a separate test step. On some Unix platforms,
cp has switches such as
-i that can be used to do the same
thing less verbosely, but you should not rely on these without
verifying that the right exit status is returned. (In
particular, GNU cp will return
status zero when -i is used and the
target file already exists, which is not the desired behavior.)

While designing your archiving setup, consider what will
happen if the archive command fails repeatedly because some
aspect requires operator intervention or the archive runs out
of space. For example, this could occur if you write to tape
without an autochanger; when the tape fills, nothing further
can be archived until the tape is swapped. You should ensure
that any error condition or request to a human operator is
reported appropriately so that the situation can be resolved
reasonably quickly. The pg_wal/
directory will continue to fill with WAL segment files until
the situation is resolved. (If the file system containing
pg_wal/ fills up, PostgreSQL will do a PANIC shutdown. No
committed transactions will be lost, but the database will
remain offline until you free some space.)

The speed of the archiving command is unimportant as long as
it can keep up with the average rate at which your server
generates WAL data. Normal operation continues even if the
archiving process falls a little behind. If archiving falls
significantly behind, this will increase the amount of data
that would be lost in the event of a disaster. It will also
mean that the pg_wal/ directory
will contain large numbers of not-yet-archived segment files,
which could eventually exceed available disk space. You are
advised to monitor the archiving process to ensure that it is
working as you intend.

In writing your archive command, you should assume that the
file names to be archived can be up to 64 characters long and
can contain any combination of ASCII letters, digits, and dots.
It is not necessary to preserve the original relative path
(%p) but it is necessary to
preserve the file name (%f).

Note that although WAL archiving will allow you to restore
any modifications made to the data in your PostgreSQL database, it will not restore
changes made to configuration files (that is, postgresql.conf, pg_hba.conf and pg_ident.conf), since those are edited
manually rather than through SQL operations. You might wish to
keep the configuration files in a location that will be backed
up by your regular file system backup procedures. See Section 19.2 for how to
relocate the configuration files.

The archive command is only invoked on completed WAL
segments. Hence, if your server generates only little WAL
traffic (or has slack periods where it does so), there could be
a long delay between the completion of a transaction and its
safe recording in archive storage. To put a limit on how old
unarchived data can be, you can set archive_timeout
to force the server to switch to a new WAL segment file at
least that often. Note that archived files that are archived
early due to a forced switch are still the same length as
completely full files. It is therefore unwise to set a very
short archive_timeout — it will
bloat your archive storage. archive_timeout settings of a minute or so are
usually reasonable.

Also, you can force a segment switch manually with
pg_switch_wal if you want to
ensure that a just-finished transaction is archived as soon as
possible. Other utility functions related to WAL management are
listed in Table 9.79.

When wal_level is minimal some SQL commands are optimized to
avoid WAL logging, as described in Section 14.4.7.
If archiving or streaming replication were turned on during
execution of one of these statements, WAL would not contain
enough information for archive recovery. (Crash recovery is
unaffected.) For this reason, wal_level can only be changed at server start.
However, archive_command can be
changed with a configuration file reload. If you wish to
temporarily stop archiving, one way to do it is to set
archive_command to the empty
string (''). This will cause WAL
files to accumulate in pg_wal/
until a working archive_command is
re-established.

25.3.2. Making a Base Backup

The easiest way to perform a base backup is to use the
pg_basebackup tool. It can create a
base backup either as regular files or as a tar archive. If
more flexibility than pg_basebackup can provide is
required, you can also make a base backup using the low level
API (see Section 25.3.3).

It is not necessary to be concerned about the amount of time
it takes to make a base backup. However, if you normally run
the server with full_page_writes
disabled, you might notice a drop in performance while the
backup runs since full_page_writes
is effectively forced on during backup mode.

To make use of the backup, you will need to keep all the WAL
segment files generated during and after the file system
backup. To aid you in doing this, the base backup process
creates a backup history file that
is immediately stored into the WAL archive area. This file is
named after the first WAL segment file that you need for the
file system backup. For example, if the starting WAL file is
0000000100001234000055CD the
backup history file will be named something like 0000000100001234000055CD.007C9330.backup. (The
second part of the file name stands for an exact position
within the WAL file, and can ordinarily be ignored.) Once you
have safely archived the file system backup and the WAL segment
files used during the backup (as specified in the backup
history file), all archived WAL segments with names numerically
less are no longer needed to recover the file system backup and
can be deleted. However, you should consider keeping several
backup sets to be absolutely certain that you can recover your
data.

The backup history file is just a small text file. It
contains the label string you gave to pg_basebackup, as well as the
starting and ending times and WAL segments of the backup. If
you used the label to identify the associated dump file, then
the archived history file is enough to tell you which dump file
to restore.

Since you have to keep around all the archived WAL files
back to your last base backup, the interval between base
backups should usually be chosen based on how much storage you
want to expend on archived WAL files. You should also consider
how long you are prepared to spend recovering, if recovery
should be necessary — the system will have to replay all those
WAL segments, and that could take awhile if it has been a long
time since the last base backup.

25.3.3. Making a Base Backup Using
the Low Level API

The procedure for making a base backup using the low level
APIs contains a few more steps than the pg_basebackup method, but is
relatively simple. It is very important that these steps are
executed in sequence, and that the success of a step is
verified before proceeding to the next step.

Low level base backups can be made in a non-exclusive or an
exclusive way. The non-exclusive method is recommended and the
exclusive one is deprecated and will eventually be removed.

25.3.3.1. Making a non-exclusive
low level backup

A non-exclusive low level backup is one that allows other
concurrent backups to be running (both those started using
the same backup API and those started using pg_basebackup).

Ensure that WAL archiving is enabled and
working.

Connect to the server (it does not matter which
database) as a user with rights to run pg_start_backup
(superuser, or a user who has been granted EXECUTE on
the function) and issue the command:

SELECT pg_start_backup('label', false, false);

where label is any
string you want to use to uniquely identify this backup
operation. The connection calling pg_start_backup must be maintained
until the end of the backup, or the backup will be
automatically aborted.

By default, pg_start_backup can take a long time
to finish. This is because it performs a checkpoint,
and the I/O required for the checkpoint will be spread
out over a significant period of time, by default half
your inter-checkpoint interval (see the configuration
parameter
checkpoint_completion_target). This is usually what
you want, because it minimizes the impact on query
processing. If you want to start the backup as soon as
possible, change the second parameter to true, which will issue an immediate
checkpoint using as much I/O as available.

The third parameter being false tells pg_start_backup to initiate a
non-exclusive base backup.

Perform the backup, using any convenient
file-system-backup tool such as tar or cpio (not pg_dump or pg_dumpall). It is neither
necessary nor desirable to stop normal operation of the
database while you do this. See Section 25.3.3.3
for things to consider during this backup.

In the same connection as before, issue the
command:

SELECT * FROM pg_stop_backup(false, true);

This terminates backup mode. On a primary, it also
performs an automatic switch to the next WAL segment.
On a standby, it is not possible to automatically
switch WAL segments, so you may wish to run
pg_switch_wal on the
primary to perform a manual switch. The reason for the
switch is to arrange for the last WAL segment file
written during the backup interval to be ready to
archive.

The pg_stop_backup
will return one row with three values. The second of
these fields should be written to a file named
backup_label in the root
directory of the backup. The third field should be
written to a file named tablespace_map unless the field is
empty. These files are vital to the backup working, and
must be written without modification.

Once the WAL segment files active during the backup
are archived, you are done. The file identified by
pg_stop_backup's first
return value is the last segment that is required to
form a complete set of backup files. On a primary, if
archive_mode is enabled
and the wait_for_archive
parameter is true,
pg_stop_backup does not
return until the last segment has been archived. On a
standby, archive_mode must
be always in order for
pg_stop_backup to wait.
Archiving of these files happens automatically since
you have already configured archive_command. In most cases this
happens quickly, but you are advised to monitor your
archive system to ensure there are no delays. If the
archive process has fallen behind because of failures
of the archive command, it will keep retrying until the
archive succeeds and the backup is complete. If you
wish to place a time limit on the execution of
pg_stop_backup, set an
appropriate statement_timeout value, but make note
that if pg_stop_backup
terminates because of this your backup may not be
valid.

If the backup process monitors and ensures that all
WAL segment files required for the backup are
successfully archived then the wait_for_archive parameter (which
defaults to true) can be set to false to have
pg_stop_backup return as
soon as the stop backup record is written to the WAL.
By default, pg_stop_backup will wait until all
WAL has been archived, which can take some time. This
option must be used with caution: if WAL archiving is
not monitored correctly then the backup might not
include all of the WAL files and will therefore be
incomplete and not able to be restored.

25.3.3.2. Making an exclusive
low level backup

The process for an exclusive backup is mostly the same as
for a non-exclusive one, but it differs in a few key steps.
This type of backup can only be taken on a primary and does
not allow concurrent backups. Prior to PostgreSQL 9.6, this was the only
low-level method available, but it is now recommended that
all users upgrade their scripts to use non-exclusive backups
if possible.

Ensure that WAL archiving is enabled and
working.

Connect to the server (it does not matter which
database) as a user with rights to run pg_start_backup
(superuser, or a user who has been granted EXECUTE on
the function) and issue the command:

SELECT pg_start_backup('label');

where label is any
string you want to use to uniquely identify this backup
operation. pg_start_backup creates a backup label file, called backup_label, in the cluster
directory with information about your backup, including
the start time and label string. The function also
creates a tablespace map
file, called tablespace_map, in the cluster
directory with information about tablespace symbolic
links in pg_tblspc/ if
one or more such link is present. Both files are
critical to the integrity of the backup, should you
need to restore from it.

By default, pg_start_backup can take a long time
to finish. This is because it performs a checkpoint,
and the I/O required for the checkpoint will be spread
out over a significant period of time, by default half
your inter-checkpoint interval (see the configuration
parameter
checkpoint_completion_target). This is usually what
you want, because it minimizes the impact on query
processing. If you want to start the backup as soon as
possible, use:

SELECT pg_start_backup('label', true);

This forces the checkpoint to be done as quickly as
possible.

Perform the backup, using any convenient
file-system-backup tool such as tar or cpio (not pg_dump or pg_dumpall). It is neither
necessary nor desirable to stop normal operation of the
database while you do this. See Section 25.3.3.3
for things to consider during this backup.

Note that if the server crashes during the backup it
may not be possible to restart until the backup_label file has been manually
deleted from the PGDATA
directory.

Again connect to the database as a user with rights
to run pg_stop_backup (superuser, or a user who has
been granted EXECUTE on the function), and issue the
command:

SELECT pg_stop_backup();

This function terminates backup mode and performs an
automatic switch to the next WAL segment. The reason
for the switch is to arrange for the last WAL segment
written during the backup interval to be ready to
archive.

Once the WAL segment files active during the backup
are archived, you are done. The file identified by
pg_stop_backup's result
is the last segment that is required to form a complete
set of backup files. If archive_mode is enabled, pg_stop_backup does not return until
the last segment has been archived. Archiving of these
files happens automatically since you have already
configured archive_command. In most cases this
happens quickly, but you are advised to monitor your
archive system to ensure there are no delays. If the
archive process has fallen behind because of failures
of the archive command, it will keep retrying until the
archive succeeds and the backup is complete. If you
wish to place a time limit on the execution of
pg_stop_backup, set an
appropriate statement_timeout value, but make note
that if pg_stop_backup
terminates because of this your backup may not be
valid.

25.3.3.3. Backing up the data
directory

Some file system backup tools emit warnings or errors if
the files they are trying to copy change while the copy
proceeds. When taking a base backup of an active database,
this situation is normal and not an error. However, you need
to ensure that you can distinguish complaints of this sort
from real errors. For example, some versions of rsync return a separate exit code for
“vanished source
files”, and you can write a driver script to
accept this exit code as a non-error case. Also, some
versions of GNU tar return
an error code indistinguishable from a fatal error if a file
was truncated while tar was
copying it. Fortunately, GNU tar versions 1.16 and later exit with 1
if a file was changed during the backup, and 2 for other
errors. With GNU tar version
1.23 and later, you can use the warning options --warning=no-file-changed
--warning=no-file-removed to hide the related warning
messages.

Be certain that your backup includes all of the files
under the database cluster directory (e.g., /usr/local/pgsql/data). If you are using
tablespaces that do not reside underneath this directory, be
careful to include them as well (and be sure that your backup
archives symbolic links as links, otherwise the restore will
corrupt your tablespaces).

You should, however, omit from the backup the files within
the cluster's pg_wal/
subdirectory. This slight adjustment is worthwhile because it
reduces the risk of mistakes when restoring. This is easy to
arrange if pg_wal/ is a
symbolic link pointing to someplace outside the cluster
directory, which is a common setup anyway for performance
reasons. You might also want to exclude postmaster.pid and postmaster.opts, which record information
about the running postmaster, not about the postmaster which will eventually use
this backup. (These files can confuse pg_ctl.)

It is often a good idea to also omit from the backup the
files within the cluster's pg_replslot/ directory, so that replication
slots that exist on the master do not become part of the
backup. Otherwise, the subsequent use of the backup to create
a standby may result in indefinite retention of WAL files on
the standby, and possibly bloat on the master if hot standby
feedback is enabled, because the clients that are using those
replication slots will still be connecting to and updating
the slots on the master, not the standby. Even if the backup
is only intended for use in creating a new master, copying
the replication slots isn't expected to be particularly
useful, since the contents of those slots will likely be
badly out of date by the time the new master comes on
line.

The contents of the directories pg_dynshmem/, pg_notify/, pg_serial/, pg_snapshots/, pg_stat_tmp/, and pg_subtrans/ (but not the directories
themselves) can be omitted from the backup as they will be
initialized on postmaster startup. If stats_temp_directory
is set and is under the data directory then the contents of
that directory can also be omitted.

Any file or directory beginning with pgsql_tmp can be omitted from the backup.
These files are removed on postmaster start and the
directories will be recreated as needed.

pg_internal.init files can
be omitted from the backup whenever a file of that name is
found. These files contain relation cache data that is always
rebuilt when recovering.

The backup label file includes the label string you gave
to pg_start_backup, as well as
the time at which pg_start_backup was run, and the name of
the starting WAL file. In case of confusion it is therefore
possible to look inside a backup file and determine exactly
which backup session the dump file came from. The tablespace
map file includes the symbolic link names as they exist in
the directory pg_tblspc/ and
the full path of each symbolic link. These files are not
merely for your information; their presence and contents are
critical to the proper operation of the system's recovery
process.

It is also possible to make a backup while the server is
stopped. In this case, you obviously cannot use pg_start_backup or pg_stop_backup, and you will therefore be
left to your own devices to keep track of which backup is
which and how far back the associated WAL files go. It is
generally better to follow the continuous archiving procedure
above.

25.3.4. Recovering Using a
Continuous Archive Backup

Okay, the worst has happened and you need to recover from
your backup. Here is the procedure:

Stop the server, if it's running.

If you have the space to do so, copy the whole cluster
data directory and any tablespaces to a temporary
location in case you need them later. Note that this
precaution will require that you have enough free space
on your system to hold two copies of your existing
database. If you do not have enough space, you should at
least save the contents of the cluster's pg_wal subdirectory, as it might
contain logs which were not archived before the system
went down.

Remove all existing files and subdirectories under the
cluster data directory and under the root directories of
any tablespaces you are using.

Restore the database files from your file system
backup. Be sure that they are restored with the right
ownership (the database system user, not root!) and with the right permissions.
If you are using tablespaces, you should verify that the
symbolic links in pg_tblspc/ were correctly restored.

Remove any files present in pg_wal/; these came from the file
system backup and are therefore probably obsolete rather
than current. If you didn't archive pg_wal/ at all, then recreate it with
proper permissions, being careful to ensure that you
re-establish it as a symbolic link if you had it set up
that way before.

If you have unarchived WAL segment files that you
saved in step 2, copy them into pg_wal/. (It is best to copy them, not
move them, so you still have the unmodified files if a
problem occurs and you have to start over.)

Create a recovery command file recovery.conf in the cluster data
directory (see Chapter 27).
You might also want to temporarily modify pg_hba.conf to prevent ordinary users
from connecting until you are sure the recovery was
successful.

Start the server. The server will go into recovery
mode and proceed to read through the archived WAL files
it needs. Should the recovery be terminated because of an
external error, the server can simply be restarted and it
will continue recovery. Upon completion of the recovery
process, the server will rename recovery.conf to recovery.done (to prevent accidentally
re-entering recovery mode later) and then commence normal
database operations.

Inspect the contents of the database to ensure you
have recovered to the desired state. If not, return to
step 1. If all is well, allow your users to connect by
restoring pg_hba.conf to
normal.

The key part of all this is to set up a recovery
configuration file that describes how you want to recover and
how far the recovery should run. You can use recovery.conf.sample (normally located in the
installation's share/ directory)
as a prototype. The one thing that you absolutely must specify
in recovery.conf is the
restore_command, which tells
PostgreSQL how to retrieve
archived WAL file segments. Like the archive_command, this is a shell command
string. It can contain %f, which
is replaced by the name of the desired log file, and
%p, which is replaced by the path
name to copy the log file to. (The path name is relative to the
current working directory, i.e., the cluster's data directory.)
Write %% if you need to embed an
actual % character in the command.
The simplest useful command is something like:

restore_command = 'cp /mnt/server/archivedir/%f %p'

which will copy previously archived WAL segments from the
directory /mnt/server/archivedir.
Of course, you can use something much more complicated, perhaps
even a shell script that requests the operator to mount an
appropriate tape.

It is important that the command return nonzero exit status
on failure. The command will be called requesting files that
are not present in the archive; it must return nonzero when so
asked. This is not an error condition. An exception is that if
the command was terminated by a signal (other than SIGTERM, which is used as part of a
database server shutdown) or an error by the shell (such as
command not found), then recovery will abort and the server
will not start up.

Not all of the requested files will be WAL segment files;
you should also expect requests for files with a suffix of
.history. Also be aware that the
base name of the %p path will be
different from %f; do not expect
them to be interchangeable.

WAL segments that cannot be found in the archive will be
sought in pg_wal/; this allows
use of recent un-archived segments. However, segments that are
available from the archive will be used in preference to files
in pg_wal/.

Normally, recovery will proceed through all available WAL
segments, thereby restoring the database to the current point
in time (or as close as possible given the available WAL
segments). Therefore, a normal recovery will end with a
“file not
found” message, the exact text of the error
message depending upon your choice of restore_command. You may also see an error
message at the start of recovery for a file named something
like 00000001.history. This is
also normal and does not indicate a problem in simple recovery
situations; see Section 25.3.5 for
discussion.

If you want to recover to some previous point in time (say,
right before the junior DBA dropped your main transaction
table), just specify the required stopping point in
recovery.conf. You can specify
the stop point, known as the “recovery target”, either by date/time,
named restore point or by completion of a specific transaction
ID. As of this writing only the date/time and named restore
point options are very usable, since there are no tools to help
you identify with any accuracy which transaction ID to use.

Note

The stop point must be after the ending time of the base
backup, i.e., the end time of pg_stop_backup. You cannot use a base
backup to recover to a time when that backup was in progress.
(To recover to such a time, you must go back to your previous
base backup and roll forward from there.)

If recovery finds corrupted WAL data, recovery will halt at
that point and the server will not start. In such a case the
recovery process could be re-run from the beginning, specifying
a “recovery
target” before the point of corruption so that
recovery can complete normally. If recovery fails for an
external reason, such as a system crash or if the WAL archive
has become inaccessible, then the recovery can simply be
restarted and it will restart almost from where it failed.
Recovery restart works much like checkpointing in normal
operation: the server periodically forces all its state to
disk, and then updates the pg_control file to indicate that the
already-processed WAL data need not be scanned again.

25.3.5. Timelines

The ability to restore the database to a previous point in
time creates some complexities that are akin to science-fiction
stories about time travel and parallel universes. For example,
in the original history of the database, suppose you dropped a
critical table at 5:15PM on Tuesday evening, but didn't realize
your mistake until Wednesday noon. Unfazed, you get out your
backup, restore to the point-in-time 5:14PM Tuesday evening,
and are up and running. In this history of the database
universe, you never dropped the table. But suppose you later
realize this wasn't such a great idea, and would like to return
to sometime Wednesday morning in the original history. You
won't be able to if, while your database was up-and-running, it
overwrote some of the WAL segment files that led up to the time
you now wish you could get back to. Thus, to avoid this, you
need to distinguish the series of WAL records generated after
you've done a point-in-time recovery from those that were
generated in the original database history.

To deal with this problem, PostgreSQL has a notion of timelines. Whenever an archive recovery
completes, a new timeline is created to identify the series of
WAL records generated after that recovery. The timeline ID
number is part of WAL segment file names so a new timeline does
not overwrite the WAL data generated by previous timelines. It
is in fact possible to archive many different timelines. While
that might seem like a useless feature, it's often a lifesaver.
Consider the situation where you aren't quite sure what
point-in-time to recover to, and so have to do several
point-in-time recoveries by trial and error until you find the
best place to branch off from the old history. Without
timelines this process would soon generate an unmanageable
mess. With timelines, you can recover to any prior state, including states in
timeline branches that you abandoned earlier.

Every time a new timeline is created, PostgreSQL creates a “timeline history”
file that shows which timeline it branched off from and when.
These history files are necessary to allow the system to pick
the right WAL segment files when recovering from an archive
that contains multiple timelines. Therefore, they are archived
into the WAL archive area just like WAL segment files. The
history files are just small text files, so it's cheap and
appropriate to keep them around indefinitely (unlike the
segment files which are large). You can, if you like, add
comments to a history file to record your own notes about how
and why this particular timeline was created. Such comments
will be especially valuable when you have a thicket of
different timelines as a result of experimentation.

The default behavior of recovery is to recover along the
same timeline that was current when the base backup was taken.
If you wish to recover into some child timeline (that is, you
want to return to some state that was itself generated after a
recovery attempt), you need to specify the target timeline ID
in recovery.conf. You cannot
recover into timelines that branched off earlier than the base
backup.

25.3.6. Tips and Examples

Some tips for configuring continuous archiving are given
here.

25.3.6.1. Standalone Hot
Backups

It is possible to use PostgreSQL's backup facilities to
produce standalone hot backups. These are backups that cannot
be used for point-in-time recovery, yet are typically much
faster to backup and restore than pg_dump dumps. (They are also much
larger than pg_dump dumps,
so in some cases the speed advantage might be negated.)

As with base backups, the easiest way to produce a
standalone hot backup is to use the pg_basebackup tool. If you include
the -X parameter when calling
it, all the write-ahead log required to use the backup will
be included in the backup automatically, and no special
action is required to restore the backup.

If more flexibility in copying the backup files is needed,
a lower level process can be used for standalone hot backups
as well. To prepare for low level standalone hot backups,
make sure wal_level is set to
replica or higher, archive_mode to on, and set up an archive_command that performs archiving only
when a switch file
exists. For example:

The switch file /var/lib/pgsql/backup_in_progress is
created first, enabling archiving of completed WAL files to
occur. After the backup the switch file is removed. Archived
WAL files are then added to the backup so that both base
backup and all required WAL files are part of the same
tar file. Please remember to
add error handling to your backup scripts.

25.3.6.2. Compressed Archive
Logs

If archive storage size is a concern, you can use
gzip to compress the archive
files:

archive_command = 'gzip < %p > /var/lib/pgsql/archive/%f'

You will then need to use gunzip during recovery:

restore_command = 'gunzip < /mnt/server/archivedir/%f > %p'

25.3.6.3. archive_command Scripts

Many people choose to use scripts to define their
archive_command, so that their
postgresql.conf entry looks
very simple:

archive_command = 'local_backup_script.sh "%p" "%f"'

Using a separate script file is advisable any time you
want to use more than a single command in the archiving
process. This allows all complexity to be managed within the
script, which can be written in a popular scripting language
such as bash or perl.

Examples of requirements that might be solved within a
script include:

Copying data to secure off-site data storage

Batching WAL files so that they are transferred
every three hours, rather than one at a time

Interfacing with other backup and recovery
software

Interfacing with monitoring software to report
errors

Tip

When using an archive_command script, it's desirable to
enable logging_collector.
Any messages written to stderr from the script will then appear
in the database server log, allowing complex configurations
to be diagnosed easily if they fail.

25.3.7. Caveats

At this writing, there are several limitations of the
continuous archiving technique. These will probably be fixed in
future releases:

If a CREATE DATABASE command is
executed while a base backup is being taken, and then the
template database that the CREATE
DATABASE copied is modified while the base backup
is still in progress, it is possible that recovery will
cause those modifications to be propagated into the
created database as well. This is of course undesirable.
To avoid this risk, it is best not to modify any template
databases while taking a base backup.

CREATE TABLESPACE commands are
WAL-logged with the literal absolute path, and will
therefore be replayed as tablespace creations with the
same absolute path. This might be undesirable if the log
is being replayed on a different machine. It can be
dangerous even if the log is being replayed on the same
machine, but into a new data directory: the replay will
still overwrite the contents of the original tablespace.
To avoid potential gotchas of this sort, the best
practice is to take a new base backup after creating or
dropping tablespaces.

It should also be noted that the default WAL format is fairly bulky since it
includes many disk page snapshots. These page snapshots are
designed to support crash recovery, since we might need to fix
partially-written disk pages. Depending on your system hardware
and software, the risk of partial writes might be small enough
to ignore, in which case you can significantly reduce the total
volume of archived logs by turning off page snapshots using the
full_page_writes
parameter. (Read the notes and warnings in Chapter 30
before you do so.) Turning off page snapshots does not prevent
use of the logs for PITR operations. An area for future
development is to compress archived WAL data by removing
unnecessary page copies even when full_page_writes is on. In the meantime,
administrators might wish to reduce the number of page
snapshots included in WAL by increasing the checkpoint interval
parameters as much as feasible.

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